Pledged securities owned and pledged as collateral amount eligible to be repledged by counterparty carrying value.

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Percentage of repurchase of class A common stock.

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Face amount or stated value per share of common stock.

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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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The number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

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The remaining number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

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Number of shares that have been repurchased and retired during the period.

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Share Capital - Additional Information (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 05, 2017	Sep. 15, 2015
Class of Stock [Line Items]
Amounts pledged	$ 518.0				$ 655.7				$ 518.0	$ 655.7
Preferred stock, authorized	50,000,000								50,000,000
Preferred stock, par value	$ 0.001								$ 0.001
Preferred stock, issued	0								0
Dividends per share	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.44	$ 0.44	$ 440.00
Common Class A [Member]
Class of Stock [Line Items]
Common stock, authorized	50,000,000				50,000,000				50,000,000	50,000,000
Common stock, par value	$ 0.001				$ 0.001				$ 0.001	$ 0.001
Common Stock, outstanding	12,941,809				13,139,203				12,941,809	13,139,203	13,261,095		13,348,369
Common Stock, Shares, Issued	12,941,809				13,139,203				12,941,809	13,139,203
Repurchase and cancelled stock									236,122	450,350
Dividends per share									$ 0.44	$ 0.44	$ 0.44
Class B Stock
Class of Stock [Line Items]
Common stock, authorized	99,665				99,665				99,665	99,665
Common stock, par value	$ 0.001				$ 0.001				$ 0.001	$ 0.001
Common Stock, outstanding	99,665				99,665				99,665	99,665
Common Stock, Shares, Issued	99,665				99,665				99,665	99,665
Dividends per share									$ 0.44	$ 0.44	$ 0.44
Previous Program [Member] \| Common Class A [Member]
Class of Stock [Line Items]
Repurchase of class A common stock													665,000
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	40,734								40,734
New Program [Member] \| Common Class A [Member]
Class of Stock [Line Items]
Repurchase of class A common stock												650,000
Stock Repurchase Program Percentage Of Shares Repurchase Of Outstanding Share												5.00%
Stock Repurchased and Retired During Period, Shares									236,122
Stock Repurchased and Retired During Period, Value									$ 5.9
Stock Repurchased and Retired During Period, Per Share									$ 24.96
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	272,784								272,784			690,734
Common Stock, Shares, Issued												13,178,571